Apparatus for spinning low viscosity materials

ABSTRACT

APPARATUS IS PROVIDED FOR SPINNING FINE FILAMENTS FROM LOW VISCOSITY MOLTEN MATERIALS, E.G., METAL OR METAL OXIDES. THE APPARATUS COMPRISES A CRUCIBLE CONTAINING MOLTEN MATERIAL HAVING AN ORIFICE THERETHROUGH AND AN ELONGATED TUBULAR EXTENSION POSITIONED TO EXTEND FROM THE ORIFICE DOWN AWAY FROM THE BOTTOM OF THE CRUCIBLE. THE RADIUS AND LENGTH OF THE ELONGATED MEMBER ARE DEPENDENT ON THE ORIFICE RADIUS, THE MELT DENSITY AND SURFACE TENSION OF THE MOLTEN MATERIAL AND ON THE EXTRUSION VELOCITY. THE TUBULAR EXTENSION SERVES TO OVERCOME THE PROBLEM OF WETTING WHICH OTHERWISE RESULTS IN THE FORMATION OF DROPS OF MATERIAL. THE USE OF THE TUBULAR EXTENSION RESULTS IN FREE-JET EXTRUSION OF THE LOW VISCOSITY MATERIAL.

Oct. 26, 1971 p, so ETAL 3,614,808

APPARATUS FOR SPINNING LOW VISCOSITY MATERIALS Filed Nov. 6, 1968 INVENTORS Douglas P Harrlson By Robert E. Cunningham FIG. 3 M71 [4,

ATTORNEY United States Patent Office 3,614,808 Patented Oct. 26, 1971 APPARATUS FOR SPINNINGLOW VISCOSITY MATERIALS 1 Douglas P. Harrison, Durham, and Robert E. Cunningham, Raleigh, N.C., assignors to Monsanto Company, St. Louis, Mo.

Filed Nov. 6, 1968, Ser. No. 773,834 Int. Cl. D01d 5 0O U.S. c1. 18- -8 ss- 3 Claims ABSTRACT OF THE DISCLOSURE Apparatus is provided for spinning fine filaments from low viscosity inolten materials, e.g., metal or metal oxides. The apparatus comprises a crucible containing molten ma- Insofar as low viscosity ceramics and metal cannot be drawn or pulled away from the orifice as in the case of glasses and synthetic polymers, the melt extrusion of low viscosity materials presents difiiculties not experienced in melt extrusion of highly viscous materials.

It is an object of this invention to provide an apparatus for the continuous formation of fine filaments from low viscosity melts.

It is a further object of this invention to provide apparatus for the continuous formation of fine filaments from low viscosity melts which overcome the problems of orifice wetting.

terial having an orifice therethrough and an elongated v I The process of spinning fine filaments from low viscosity material, i.e., materials having a viscosity of less than 10 poises, and even less than 1 poise, by film stabili- These and other objects will appear in the description which follows.

In accordance with this invention there is provided apparatus for the production of fine filaments from low viscosity melts Which comprises a crucible containing a molten, low viscosity melt, an orifice in said crucible, an elongated member attached to the bottom of said crucible having an orifice which is concident with the orifice in said crucible, the radius and length of said elongated member being dependent upon the characteristics of the material being spun and upon the velocity at which it is spun.

In the drawings:

FIG. 1 is a perspective view of the apparatus of this invention.

FIG. 2 is a view of the crucible bottom equipped with an elongated member.

zation is a relatively new art. In these processes a molten FIG. 3 is a bottom view of the element illustrated in FIG. 2.

As seen in FIG. 1 a molten, low viscosity melt, is contained within crucible 2. Located in the bottom of the crucible is orifice insert 3 through which defines an orifice 5. It is preferred to employ such an orifice insert element for facility in changing the size of the orifice. When using such an element, if it is desired to spin a different size filament, the. element may be removed from the crucible frequently results in preventing the formation of a continuous stream through the orifice. The tendency of a liquid to wet a solid is measured by the contact angle, that is, the angle measured through the liquid, between the solid and the liquid surfaces. Whenever the contact angle isless than 90 the liquid has some tendency to wet the solid and the tendency becomes greater as the contact angle becomes closer to 0. When attempts were made to spin metal oxide compositions through refractory metal orifices by utilizing orifice configurations wherein the orifice exit is flush or slightly protuberant with the flat planar surface of the crucible bottom, failure resulted due to the wetting of the planar surface by the molten material being spun. The wetting prevented the formation of a stable free jet at the orifice exit since the emanating liquid became dispersed or spread over the planar surface upon contact therewith. As the material is being continuously extruded through the orifice the mass of material wetting the planar surface grows to a point where the sides of the pendent mass are vertical. The addition of the next small portion of material causes the force of gravity to exceed the surface tension and a drop of molten charge falls from the plate. However, some of the molten charge remains on the plate so that the continuous addition of extruded material causes another drop to form. This process of drop formation repeats itself thus preventing the formation of a continuous stream of material.

and replaced by another element having the desired orifice size. It is to be understood, however, that the orifice may occur through the crucible floor. An elongated hollow tubular extension 4 extends the orifice 5 below the planar surface on the underside of the crucible. Elongated tubular extension member 4 is more clearly shown in FIG. 2. As shown in FIG. 2 extension 4 has a length l and, as shown in FIG. 3, an outside radius, R, and inside or orifice radius R In FIG. 2, phantom drop 6 depicts, the configuration of the low viscosity melt material partially enveloping the tip of extension 4 in a controlled manner thuspreventing flow of said melt to the crucible bottom.

It has been discovered that the designed parameters for extension elongated member 4 are based on the need to create a free-jet. Problems of wetting will be encountered during start-up or during any interruption of spinning so that it is necessary to be able to create the jet from a wetted condition. The ability of a jet to penetrate a liquid pool or drop, or to eject it from the external surface of the orifice determines the maximum radius R of the end of the extension. When the length of extension 4, which will be discussed later, is sufficient to prevent extruded molten material from reaching the crucible floor, then, to establish a free-jet, the upward force that must be overcome is that of the surface tension acting around the circumference of the extension. In practice, the charge from the orifice has a certain penetrating power so that it is possible to penetrate the drop with a velocity less than that required to completely dislodge it. However, the construction criteria must be based on the principle that sufficient force must be furnished by the extrusion velocity to completely dislodge the drop prior to establishment of a stream. The maximum tip" 21r'yR =1rR pV (I) or the actual tip radius must be R02PV2 21/ (11) where R, is the outside radius of the elongated member 4, R is the radius of the orifice, p is the melt density of the material being spun, and 'y is its surface tension and V is the velocity. This equation expresses the maximum allowable radius of the extension in terms of the composition variables, p and 'y and the process variables R and V.

In addition to providing an extension having a radius sufliciently small to establish streaming, the length of the tip must be sufficient to prevent the molten charge from reaching the crucible floor, or the underside of insert ele ment 3, and subsequently spreading. The expression for the required length of the extension is obtained by equating the upward force of surface tension to the downward force of gravity. Again, considering the contact angle to zero, this relationship may be expressed as:

2vrR y=pV g where g is the gravitation acceleration and V is the volume of the drop. If it is assumed that the approximate shape of the drop is spherical, then this equation may be rewritten as:

where R is the radius of the drop and the length neces sary for the elongated member to prevent the droplet from reaching the base of the crucible can be expressed:

or, 0.173 cm, or 0.068 inch. While, therefore a diameter of less than 0.136 inch would be satisfiactory, for this particular run the diameter was 0.063 inch or 0.08 cm. Thus, solving for Equation V the length of the extension must be at least or 0.5466m. or 0.214 inch. his extended orifice design 'proved successful' and allowed continuous streaming through the orifice.

As stated above, the apparatus of this invention may be used to produce fine filaments of low viscosity materials. Among these materials are metal oxide based ceramics, for example alumina based ceramics, which have a particular tendency to wet refractory metals, such as molybdenum and tungstenrThe apparatus however, may be used tospin any low'viscosity-material which has such a tendency to .wet the surface of the orifice material. Forexample, metals such as zinc are known to have such a tendency. 1 i

What is claimed is: I

1. In combination with an apparatus for producing fine filaments from a low viscosity melt including a crucible for containing the low viscosity melt having a contact angle less than 90 with respect to the crucible, said crucible having a bottom through which the low viscosity melt is forced, an improvement which comprises a hollow elongated tubular extension extending downward from said orifice beneath said crucible bottom, said tubular extension having an outside radius, R, in centimeters, less than )1/a 2P: where g is the acceleration due to gravity, cm./sec.

whereby said tubular extension serves to prevent wetting of the crucible bottom. 2. The apparatus described in claim 1 wherein the low viscosity molten material is an alumina-based ceramic. 3. The apparatus of claim 1 wherein R is less than 10 mils.

References Cited UNITED STATES PATENTS 3,461,943 8/1969 Schile 164-89 2,907,082 10/1959 Pond 16489X 3,456,292 7/1969 Cobb 1s-s ss 3,002,226 10/1961 Warthen 1s s ss RICHARD J. HERBST, Primary Examiner A. L. HAVIS, Assistant Examiner US. Cl. X.R. 

